Usually, a person needing to wear spectacles and having thus a prescription filled by an ophthalmologist goes to the premise of an optician for choosing the frame of the future spectacles. The future wearer of the spectacles may try several spectacle frames and finally chooses one of the tried frames. The optician orders a pair of lenses corresponding to the prescription.
Depending on the practice of the optician, he can order the lenses using a “traditional” method or he may prefer to use the remote edging method.
With the traditional method, the lenses sent to the optician have been designed and manufactured according to optical criteria. The optician has to cut the lenses to the fit the spectacle frame the person has chosen.
With the remote edging method, the lenses sent to the optician have been designed, manufactured according to optical criteria and cut. The optician can fit the lenses directly in the spectacle frame the person has chosen.
The inner circumference of the openings of the chosen spectacle frame (e.g. the openings of the frame where ophthalmic lenses are intended to be mounted) can be measured very precisely by a measuring device, for example a mechanical sensor. More particularly, the openings of the frame include an inner groove and the characteristics of the groove (tilt angle with the openings, depth of the groove, etc.) can be measured by the mechanical sensor in a measuring room.
The measurements performed by the mechanical sensor on the chosen spectacle frame make it possible to order ophthalmic lenses which fit the chosen spectacle frame, on the one hand, and the wearer prescription, on the other hand.
According to the measurements performed by the mechanical sensor in the measuring room, the optician or the provider of ophthalmic lenses are able to:                determine the best semi-finished lenses for the wearer according to optical criteria, for example the wearer prescription;        edge and bevel the lenses to fit with the measurements performed on the chosen spectacle frame.        
In the sense of the invention a step of cutting the lenses according to a spectacle frame shape is called “edging” and a step of forming a bevel on an external edged of the lens is called “beveling.”
The lenses provider has to ensure that the provided lenses are adapted to the wearer prescription and to the chosen spectacle frame.
For example, the lenses provider has to ensure that the future lenses can effectively fit the chosen frame which may have particular openings and groove.
It will be thus understood that the measurements performed on the inner circumference openings of the chosen frame and the choice of the semi-finished lens are of great importance for the provider.
Conventionally, spectacle lenses are manufactured on request in accordance with specifications intrinsic to individual wearers. However lenses are commonly manufactured by using a limited number of semi-finished lens blanks. A semi-finished lens blank has two main faces where a face is the front face of the final lens and the other face is machined so as the optical system of the final lens fits the wearer ophthalmic prescriptions.
Semi-finished lens blanks are usually obtained by injection moulding or by casting into moulds.
The surface not to be machined of a semi-finished lens blank is conventionally called a “base-curve”.
In the frame of the present invention and according to ISO Standard 13666:1998(E/F) (Ophthalmic optics—Spectacle lenses—Vocabulary), the curvature of the front face is called a “base-curve”.
The front face of a semi-finished lens blank is usually intended to be the final front surface of the final lens and the other face is machined so as the optical system of the final lens fits the wearer ophthalmic prescriptions. Some minor machining of the front face may occur, but without modifying its curvature.
Semi-finished lens blanks are usually obtained by injection moulding or by casting into moulds. They also can be produced by machining a blank.
Manufacturers typically produce a series of semi-finished lens blanks, each with its own base curve. This “base-curve series” is a system of lens blanks that increases incrementally in surface power (e.g., +0.50 D, +2.00 D, +4.00 D, and so on).
The base-curves of a lens series serves as the starting point from which the remaining curves of the back surface will be calculated and the final lens be manufactured according to a wearer prescription (or focal power).
Each base-curve in a series is conventionally used for producing a range of prescription, as specified by the manufacturer. Manufacturers make base-curve selection charts available that provide the recommended prescription ranges for each base-curve in the series. An example of a typical base-curve selection chart is disclosed in patent document U.S. Pat. No. 6,948,816 where the base-curve series of FIGS. 23 A to C comprises five base-curves. The selection chart indicates the unique base-curve to be chosen according to a given prescription as a function of the spherical power SPH and of the cylindrical power CYL for curing an astigmatic vision. The disclosed selection chart relates to progressive addition lenses (progressive lens) in which a power continuously changes between a distance portion and a near portion. The same type of selection chart is widely used for every kind of ophthalmic lenses such as for example single lenses (spherical and/or torical), bi-focal lenses, aspherical lens, progressive lens.
The common trend is to limit the number of base-curves of a base-curve series in order to minimize the mould number, the stocking costs and inventory requirements. A standard base-curve series comprises less or equal to twenty base-curves, as for example equal or less or equal to ten, and preferably five to eight base-curves.
It has to be noticed that the calculation of the base-curve surfaces is a key point for each lens manufacturer, in particular when dealing with progressive lens where the progressive lens “model design” is an essential parameter of the base-curve surface. A progressive “model design” results of an optimization of the progressive surface so as to restore a presbyope's ability to see clearly at all distances but also to optimally respect all physiological visual functions such as foveal vision, extra-foveal vision, binocular vision and to minimize unwanted astigmatisms. Said progressive “model designs” are tested through rigorous clinical trials before being commercialized.
Conventionally, when the optician orders the lenses, the manufacturer selects a pair of semi-finished lenses. The selection of the semi-finished lens is based on optical criteria, such as the wearer prescription, optical comfort and the measured parameter of the spectacle frame are used for the edging and beveling steps.
The optical comfort may include a choice of the base curve of the front face of the semi-finished lens. Indeed, it is generally more comfortable for the wearer to keep the same front base as his previous ophthalmic lenses.
In some cases, the edging and beveling step can be preceded according not only to the measured shape of the spectacle frame but also according to esthetic criteria chosen by the wearer. For example, the esthetic criteria may implies having front face edges of the ophthalmic lens substantially abutting with front faces of the chosen spectacle frame when the ophthalmic lenses are fitted in the frame.
According to the esthetic criteria of the wearer and his prescription it is not always possible to meet both optical and esthetic criteria.
Usually the lens manufacturer will select the semi-finished lens according to the optical criteria and will grind a calculated design fitting the wearer prescription on the rear face of the semi-fined lens.
The person executing the edging and beveling steps, for example a edger or the optician, will receive the ophthalmic lens and will have to proceed to the edging and beveling steps according to the shape of the spectacle frame and to the esthetic criteria of the wearer.
Depending on the shape of the ophthalmic lens, it is not always possible to edge and bevel the ophthalmic lens according to the wearer criteria because of an improper lens shape (external shape of the lens, shapes of the front and rear surfaces of the lens, lens thickness, etc.).
For example, the bevel curve of the spectacle lens may be less steep compared with the curve of the frame, depending on the bevel position.
In some cases the frame is deformed so as to be matched with the bevel curve. However, deforming the frame can result in an increase of the frame size beyond the values calculated when the bevel position was selected, and thus in deficiency of the outer diameter or edge thickness of the lens. Furthermore, some spectacle frames are not deformable.
Therefore, in some cases the optician will receive a pair of lenses that can not be adapted to the spectacle frame.
One object of the present invention is to improve the situation.